resume-guests-state-on-host-boot should be on by default or have better documentation

[James Troup]

The current documentation for resume-guests-state-on-host-boot simply
says this:

      This option determines whether to start guests that were running
      before the host rebooted.

But it still defaults to 'false' which is not what most people want
(IMO). When I asked, I was told that there use to be bugs that would
cause VMs to start before the metadata service which meant that if
this option were set to 'true', VMs would start in a weird state.
Additionally, it was pointed out that you probably want this set to
'false' if you're using Masakari (which makes sense).

Ideally I would like to see the following happen:

 1) the bug about VMs starting before the Metadata services is either
    a) fixed/confirmed no longer relevant or b) documented in the juju
    config text for resume-guests-state-on-host-boot

 2) the Masakari interaction is either:

   a) handled via relation, i.e. if nova-compute is related to
      maskari(-monitors), it either overrides
      resume-guests-state-on-host-boot to false or goes into blocked
      state

  or

   b) is documented as something for the administrator to consider in
      juju config text